1. Field of the Invention
This invention relates generally to a system and method for reducing the chance of a fuel cell system quick stop because of a high current draw from the cathode compressor at the end of a system start-up procedure and, more particularly, to a system and method for reducing the chance of a fuel cell system quick stop because of a high current draw from the cathode compressor at the end of a system start-up procedure by gradually increasing the compressor speed command.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive maternal, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
Fuel cell system start-up procedures can sometimes be delicate if the fuel cell stack is not at or near its optimal operating temperature, especially for cold or freeze start-up procedures. When a fuel cell stack is not at or near its operating temperature just after the start-up procedure, a high power request on the fuel cell stack may cause the stack voltage to be reduced below a value that could cause a stack quick stop to occur. A more detailed discussion of this process is given as follows.
After completing the start-up procedure of a fuel cell system where reactants are flowing through the stack at a rate that is sufficient to sustain the stack voltage and supply power to the parasitic loads, certain system procedures typically increase the cathode compressor flow rate to a high speed generally to dilute the hydrogen in the system exhaust stream. In one known fuel cell system design that employs an ultracapacitor and a high voltage bus, a blocking diode separating the bus from the stack and a start-up contactor that by-passes the diode for starting up the fuel cell system, it is necessary to slow the compressor down to a very low speed in order to allow a transition from the start-up contactor to the main stack contactors. After the main contactors have closed, the system enters a run state and the compressor typically immediately receives a step request to increase the compressor flow rate a significant amount to provide anode exhaust dilution.
A problem exists in this procedure because the step change of the compressor flow rate corresponds to a large in-rush of current to the compressor motor. The stack is not always able to keep up with this large current in-rush, which can cause stack instability. This will be especially true for older stacks, and cold start-ups, which could lead to a system quick stop.
A related problem is that when a cold start is initiated and completed, the fuel cell system is not always in a completely stable state and may not be able to adequately accommodate a rapid power up transient shortly after completing the start-up procedure. During the initial period following the completion of the start-up procedure, the stack should undergo a period of conditioning before it is capable of successfully performing harsh power up-transients. conditioning is the first process by which the membranes in the stack reach an optimal humidification level whereby they are able to conduct protons efficiently. This generally takes place during normal operation once the stack has reached its operating temperature.
In another existing fuel cell system, the stack requests a step increase in a compressor flow rate immediately following the completion of a start-up procedure. Then, for a short period of time, the compressor maintains the higher flour rate as its minimum set-point, and if a high power request comes during that time that requires a higher flow rate, then a step change to the flow request takes place.